The present invention is concerned with a method for thickening various suspensions, such as, e.g., pulp slurry, waste waters, or mineral sediments, so that the sludge or slurry is passed into a basin in which a filter unit rotates partly submerged in the sludge or slurry, the thickened sludge or pulp web being gathered onto the surface of said filter from the sludge or the slurry in the basin, at which time the sludge or slurry is passed from the basin to above the sludge or slurry level. The invention is also concerned with equipment for the implementation of the method, said equipment comprising a basin for the sludge or slurry, a rotary filter unit partly submerged in the sludge or slurry, a device for passing the sludge or slurry into the basin above the level of the sludge or slurry, and a device for removing the sludge or pulp web from the filter unit.
It is well known that when, e.g., a cellulose fibre sludge or slurry is fed by of a rotary drum filter, the capacity of the filter depends on the feeding thickness of the pulp fed onto the filter. When the thickness is increased, e.g., from the value 0.5% to 1.5%, the capacity of the filter is almost doubled. For pre-thickening the pulp, a known procedure is the use of a rotary drum prethickener, which is arranged in front of the filter proper. Such a separate prethickener, however, requires a drive mechanism of its own and equipment for passing jet water into same. The space requirement of the prethickener is quite large, and it also requires a considerable additional investment.
The object of the present invention is to increase the thickness of the sludge or slurry to be fed to the filter in order to increase the capacity of the filter without a separate prethickener. The method in accordance with the invention is mainly characterized in that, in order to pre-thicken the sludge or slurry, part of the liquid contained therein is removed through a wall located beween the feeding-in point of the sludge or slurry and the surface level of the sludge or slurry. The equipment in accordance with the invention is characterized in that the wall located between the feeding-in point of the sludge or slurry and the level of the sludge or slurry wholly or partly consists of a portion of the wall provided with openings or channels.
When the equipment in accordance with the invention is used, neiter extra space nor the separate drive mechanism required by a separate drum filter is needed. In the equipment in accordance with the invention it is easy to control the feeding thickness of the sludge or slurry fed to the filter proper. It is also very easy to cleanse the equipment.